203--Remove LinkedList Elements

package LinedList;

public class RemoveLinkedListElements {
    //解法一：循环
    public ListNode removeElements(ListNode head, int val) {
        while (head!=null&&head.val==val){
            ListNode temp=head;
            head=temp.next;
            temp.next=null;
        }
        if (head==null)
            return null;
        ListNode previous=head;
        while (previous.next!=null){
            if (previous.next.val==val){
                ListNode temp=previous.next;
                previous.next=temp.next;
                temp.next=null;
            }
            else{
                previous=previous.next;
            }
        }
        return head;
    }
    //解法二，使用虚拟头节点的循环。
    public ListNode removeElements2(ListNode head, int val) {
       ListNode dummyHead=new ListNode(-1);
       dummyHead.next=head;
       ListNode previous=dummyHead;
       while (previous.next!=null){
           if (previous.next.val==val){
               ListNode temp=previous.next;
               previous.next=temp.next;
               temp.next=null;
           } else {
               previous=previous.next;
           }
       }
       return dummyHead.next;
    }
    //解法三：使用递归。
    public ListNode removeElements3(ListNode head, int val) {
        if(head==null)
            return null;
        head.next=removeElements(head.next,val);
        return head.val==val?head.next:head;
    }
}